Structures of sulfur on TiO2(110) determined by scanning tunneling microscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction

The temperature dependent adsorption of sulfur on TiO2(1 1 0) has been stud ied with X-ray photoelectron spectroscopy (XPS). scanning tunneling microsc opy (STM), and low-energy electron diffraction (LEED). Sulfur adsorbs disso ciatively at room temperature and binds to fivefold coordinated Ti atoms. U pon heating to similar to 120 degreesC, 80% of the sulfur desorbs and the S 2p peak position changes from 164.3 +/- 0.1 to 162.5 +/- 0.1 eV. This peak shift corresponds to a change of the adsorption site to the position of th e bridging oxygen atoms of TiO2(1 1 0). Further heating causes little chang e in S coverage and XPS binding energies, up to a temperature of similar to 430 degreesC where most of the S desorbs and the S 2p peak shifts back to higher binding energy. Sulfur adsorption at 150 degreesC, 200 degreesC, and 300 degreesC leads to a rich variety of structures and adsorption sites as observed with LEED and STM. At low coverages, sulfur occupies the position of the bridging oxygen atoms. At 200 degreesC these S atoms arrange in a ( 3 x 1) superstructure. For adsorption between 300 degreesC and 400 degreesC a (3 x 3) and (4 x 1) LEED pattern is observed for intermediate and satura tion coverage, respectively. Adsorption at elevated temperature reduces the substrate as indicated by a strong Ti3+ shoulder in the XPS Ti 2p(3) (2) p eak, with up to 15.6% of the total peak area for the (4 x 1) structure. STM of different coverages adsorbed at 400 degreesC indicates structural featu res consisting of two single S atoms placed next to each other along the [0 0 1] direction at the position of the in-plane oxygen atoms. The (3 x 3) a nd the (4 x 1) structure are formed by different arrangements of these S pa irs. (C) 2001 Elsevier Science B.V. All rights reserved.